I am currently in the process of designing a high-precision mixed signal PCB. I do have some experience designing simpler PCBs, and I have been doing my own research for this question. However I figured it might be better to collect input from people no doubt much more experienced than I am.
Context: I have a mixed digital/analog circuit, with my priority being keeping the analog signals as noise free as possible (its some DAC and ADC stuff where I am dealing with very very low voltages and I need to maintain a high degree of precision). I took the liberty of attaching a very rough functional block diagram to illustrate. It is also important to note that I am not dealing with any RF, high-speed digital signals or anything of the sort.
The entire board will be powered via +5V USB, which will feed directly (and interface with) a microcontroller and some other logic. The Problem: The analog section of the circuit requires ±15V, which I obtain with a dual output boost converter. I would like to maintain a level of electrical isolation between the USB side and the boost converter side. I understand I can't have complete isolation since both sections do share the same power supply.
My first instinct was to look into dedicated isolation ICs, and I have settled on a design where I isolate the +5V line with a SN6505 based isolation circuit (which funnily enough uses the same boost-converter IC in it's example application from the datasheet). This is represented by ISO1 in the diagram.
Now since I still need to communicate with some digital peripherals from the other side, I will also employ a SPI/digital isolator (probably something from Analog's ADuM line). This is represented by ISO2 in the diagram.
I am aware that splitting ground planes is very tricky to do, and there's a lot of mixed opinion. I did do research to educate myself more about current loops and return paths, proper layout and the like. Due to the isolation ICs I would not need to cross any traces over the ground cut. Additionally, there is nothing high frequency involved. This leads me to believe that the ground plane cut (indicated by the RED CUT in the diagram) would be a suitable (and not a entirely stupid) approach for this application.
If I migrate to a 4-layer board, would 2 different ground plane layers work? For example, suppose I have the stackup: SIGNAL + DGND + AGND + SIGNAL. This way I don't need to split any ground planes. I am unaware of what noise, EMI, and other implications this might have, and I have not been able to find anything substantial online about this kind of stackup (maybe because its a bad idea?).
Would it even be necessary to split the ground planes? My main rationale for wanting to split the supplies is to isolate the computer on the USB end from the boost-converter. However, if there is a smarter way to do this then by all means please let me know!
Any suggestions and criticisms for proper/smarter design would be greatly appreciated!